The present invention relates to split engine operation and particularly to an exhaust gas recirculated split engine operation with a minimum of piston pumping loss during partial cylinder mode and with a minimum of transient problems when the engine is switched from partial to full cylinder mode.
Split engine control is well known in the art as a means for providing fuel economy when a vehicle can operate at reduced engine output power by deactivating certain of the cylinders and allowing the activated cylinders to operate at their maximum efficiency. Closed loop engine control using an exhuast gas sensor as a means for generating a feedback control signal and exhaust gas recirculation (EGR) are also well known as a means for reducing the exhaust emissions. Because of the need to meet fuel savings and emission standards, these known techniques are extensively used. However, a mere combination of these techniques would result in disadvantages. For example, split engine control would produce air-diluted exhaust gases if the deactivated cylinders are allowed to suck in and pump out intake air, so that the exhaust gas sensor would produce an enrichment signal which tends to over-enrich the activated cylinders. To prevent this over-enrich problem, the air supply to the deactivated cylinders should be shut off. A simple and efficient way of shutting off the air supply is to provide a shut-off valve in the air collector chamber of the intake manifold through which the manifold branches extend to respective cylinders so that, when the valve is closed air is admitted into a group of activated cylinders while preventing it from being admitted into the other cylinders during the partial cylinder operation. This will however produce a pumping loss in the deactivated cylinders since the pistons merely reciprocate therein during the partial cylinder operation. U.S. Pat. No. 947,638 filed Oct. 2, 1978, now U.S. Pat. No. 4,201,180, issued May 6, 1980, discloses a method for overcoming the aforesaid disadvantage by recirculating the exhaust gases through the deactivated cylinders. However, during the partial cylinder operation a pressure difference tends to develop between the segmented areas of the air collector chamber of the intake manifold due to the suspension of air supply to the deactivated cylinder group, and as a result higher pressure gases rush into the lower pressure area to cause an adverse effect on engine performance when full cylinder operation is resumed.